groups_join_group( int $group_id, int $user_id )

function groups_join_group( $group_id, $user_id = 0 ) {

	if ( empty( $user_id ) )
		$user_id = bp_loggedin_user_id();

	// Check if the user has an outstanding invite. If so, delete it.
	if ( groups_check_user_has_invite( $user_id, $group_id ) )
		groups_delete_invite( $user_id, $group_id );

	// Check if the user has an outstanding request. If so, delete it.
	if ( groups_check_for_membership_request( $user_id, $group_id ) )
		groups_delete_membership_request( null, $user_id, $group_id );

	// User is already a member, just return true.
	if ( groups_is_user_member( $user_id, $group_id ) )
		return true;

	$new_member                = new BP_Groups_Member;
	$new_member->group_id      = $group_id;
	$new_member->user_id       = $user_id;
	$new_member->inviter_id    = 0;
	$new_member->is_admin      = 0;
	$new_member->user_title    = '';
	$new_member->date_modified = bp_core_current_time();
	$new_member->is_confirmed  = 1;

	if ( !$new_member->save() )
		return false;

	$bp = buddypress();

	if ( !isset( $bp->groups->current_group ) || !$bp->groups->current_group || $group_id != $bp->groups->current_group->id )
		$group = groups_get_group( $group_id );
	else
		$group = $bp->groups->current_group;

	// Record this in activity feeds.
	if ( bp_is_active( 'activity' ) ) {
		groups_record_activity( array(
			'type'    => 'joined_group',
			'item_id' => $group_id,
			'user_id' => $user_id,
		) );
	}

	/**
	 * Fires after a user joins a group.
	 *
	 * @since BuddyPress 1.0.0
	 *
	 * @param int $group_id ID of the group.
	 * @param int $user_id  ID of the user joining the group.
	 */
	do_action( 'groups_join_group', $group_id, $user_id );

	return true;
}